A spark plug used for a combustion apparatus such as an internal combustion engine includes, for example, a center electrode extending in an axis direction thereof, an insulator provided on a periphery of the center electrode, a tubular metallic shell assembled to the outside of the insulator, and a ground electrode having a proximal end portion joined to a tip end portion of the metal shell. When high voltage is applied to the center electrode, a spark is generated in a gap formed between the center electrode and the ground electrode. As a result, fuel gas is ignited.
In recent years, there has been known a technology for generating a spark by applying AC power (high frequency power) to the gap instead of high voltage to promote an improvement in ignitability (see, for example, JP-A-2009-8100).
However, a spark is generated only by high frequency power in the above technology. Accordingly, depending on the condition inside a combustion chamber, the required voltage may not be produced by high frequency power alone. Therefore, even if high frequency power is applied, a situation where a spark is not generated (what is called a misfire) is likely to occur.
Hence, there has been proposed a technology for promoting an improvement in ignitability by providing an antenna for radiating an electromagnetic wave in addition to the center electrode and the ground electrode for generating a spark, and growing a spark (plasma) generated between the electrodes by an electromagnetic wave radiated from the antenna (see, for example, JP-A-2009-38026).
In the technology described in JP-A-2009-38026, however, since an electromagnetic wave is transmitted to a spark (plasma) through a space in the technology described in JP-A-2009-38026, an efficient application of energy to the spark cannot be performed and the effect of an improvement in ignitability is therefore small. Moreover, efficient radiation of an electromagnetic wave requires minute adjustments of the size of the antenna, and the like in consideration of factors, such as the wavelength, frequency, and the like of the electromagnetic waves, which may lead to an increase in manufacturing costs.
The present invention has been made in consideration of the above circumstances, and provides an ignition system and a spark plug, which can apply energy to a spark efficiently without increasing the manufacturing cost, and dramatically improve ignitability.